Heat-resistant steel containing Cr is used in exhaust system members such as exhaust manifolds and mufflers of vehicles, since the members require a high temperature strength and oxidation resistance. Since these exhaust system members may be manufactured through press working from a steel sheet or through various formings after pipe working of a steel sheet, cold rolled steel sheets as a raw material are required to have formability.
With an increase in the temperature of exhaust gas, the operating temperature of the members is also increased every year, and it is necessary to increase the high temperature strength and the like by increasing the added amounts of alloys such as Cr, Mo, and Nb. However, in the case where the amounts of added elements are increased, the workability of a raw material steel sheet is decreased in a simple manufacturing method, and thus press forming may not be performed on members having a complicated shape.
In order to improve the Lankford value (r-value) which is an index of workability of ferritic stainless steel sheets, it is effective to increase a cold rolling reduction ratio. However, since a relatively thick cold rolled steel sheet (approximately 1.5 mm to 2.5 mm) is used as a raw material for the above-described exhaust system members, there is a problem in that a sufficient cold rolling reduction ratio cannot be secured in the current manufacturing process in which the raw material thickness is regulated to a certain degree when performing cold rolling.
In order to solve this problem, a component and a manufacturing method have been devised for improving an r-value which is an index of press formability without deteriorating high temperature characteristics,
Patent Document 1 discloses component adjustment to improve workability of conventional heat-resistant ferritic stainless steel sheets, but with this, a problem occurs such as press cracking in thick materials having a relatively low cold rolling reduction ratio.
In Patent Document 2, in order to improve the r-value, the most appropriate annealing temperature of a hot rolled sheet is specified based on a relationship of an annealing temperature of a hot rolled sheet to a hot finish rolling start temperature, a hot finish rolling end temperature, and an Nb content. However, with this, sufficient workability may not be obtained according to influences of other elements (C, N, Cr, Mo, and the like) involved especially in Nb-based precipitates.
Patent Document 3 discloses a method of subjecting a hot rolled sheet to aging for 1 hour or longer, but in this case, there is a disadvantage in that the industrial manufacturing efficiency is greatly reduced.
Patent Document 4 discloses a technology for obtaining a Cr-containing heat-resistant steel sheet having a high r-value in which conditions for hot rolling and annealing of a hot rolled sheet are specified to control the crystal orientation of a center layer in a sheet thickness direction. However, since the r-value is not determined only with the crystal orientation of the center layer in a sheet thickness direction of the product, sufficient workability may not be obtained. In addition, since a heating temperature of a slab in hot rolling is in a range of 1,000° C. to 1,150° C. which is low, there is a problem such as surface scratches.
Patent Document 5 discloses a technology of specifying the crystal orientation in a region from an outermost layer to a depth of one quarter of a sheet thickness in a ferritic stainless steel sheet for an exhaust component having excellent workability. This technique increases the r-value and total elongation in a direction at an angle of 45° with respect to a rolling direction, and the technique is characterized in that annealing of a hot rolled sheet is omitted in the manufacturing method. However, in the case where only the r-value in the direction at an angle of 45° is increased, press formability is not satisfied, and in the case where annealing of a hot rolled sheet is omitted, surface defects which are called ridging cause a problem in press working and there is a problem in manufacturability such as surface scratches.